fix(gatsby-plugin-mdx): display better error message for MDX html field in development (#19111)

Author: vladar

packages/gatsby-plugin-mdx/gatsby/source-nodes.js

@@ -184,7 +184,8 @@ module.exports = (
             return html
           } catch (e) {
             reporter.error(
-              `Error querying the \`html\` field. This field is intended for use with RSS feed generation.
+              `gatsby-plugin-mdx: Error querying the \`html\` field.
+This field is intended for use with RSS feed generation.
 If you're trying to use it in application-level code, try querying for \`Mdx.body\` instead.
 Original error:
 ${e}`

packages/gatsby-plugin-mdx/utils/render-html.js

@@ -155,9 +155,8 @@ exports.mdxHTMLLoader = ({ cache, reporter, store }) =>
                 keys.map(({ body }) =>
                   renderMdxBody
                     ? renderMdxBody(body)
-                    : reporter.error(
-                        `gatsby-plugin-mdx: renderMdxBody was unavailable when rendering html.
->> This is a bug.`
+                    : new Error(
+                        `renderMdxBody was unavailable when rendering html.`
                       )
                 )
               )